The present invention relates to an electrophotographic process for forming a plurality of duplicated copies of an original by repeated development and transfer of the same and single electrostatic latent image of the original once formed on an electrophotographic photosensitive member.
Such an electrophotographic process is sometimes called a retention-type electrophotographic process and has been practiced in various kinds of duplicating machines. In this process, in order to obtain a plurality of copies having a high image quality, it is essential retain a latent image once formed on the photosensitive member for a long time period during which a plurality of copies are formed. Various methods have been proposed for retaining the latent image and avoiding deterioration thereof. However, in almost all known retention-type duplicating processes, the latent image deteriorates due to a leakage of electrostatic charge forming the latent image via a developing agent, an injection of electrostatic charge into the latent image via record papers and incompleteness of characteristics of the charge retaining member. Therefore, the electrostatic latent image is greatly impaired as the duplication process progresses and the image quality of the copies progressively decreases. There have been proposed various improvements for limiting the deterioration of the charge image as much as possible. But, heretofore it has been impossible to decrease the deterioration sufficiently.
Furthermore, in Japanese Patent Application Laid-open Publication No. 55,956/71, there is disclosed a method for restoring the deteriorated latent image during the duplication of a plurality of copies. In this known method, use is made of a photosensitive member in which a plurality of photoconductive layers having different spectral sensitivities and having different natural discharge properties (dark decay) are superimposed in a given order, and electrostatic latent images having opposite polarities are formed in dark across these photoconductive layers. When the latent image of one polarity is decays, the latent image of the other polarity decays correspondingly, whereby the whole latent image potential is maintained at an apparently constant level. In this manner, the latent image can be retained for a long time by cancelling potential leakage due to the development.
However, since such a method for retaining the latent image utilizes a photosensitive member having a special construction including photoconductive layers of different spectral sensitivities and one of the photoconductive layers must have a rectifying property, it can be applied only to a special electrophotographic process. That is to say, said known method has a drawback that the steps of duplication and printing are limited to special ones. Further, the variation of the potential of the latent image formed on the photosensitive member is determined by the dark decay of the photoconductors and therefore, there is another drawback that the image density might be changed during the formation of a number of copies.